Fix `handle_existing_function`

[akihiro17]

related to https://github.com/users/Ph0enixKM/projects/2/views/1?pane=issue&itemId=64542562

I think the handle_existing_function only needs to check if the function already exists.

script

fun Foo() {
    echo "Hello"
}

Foo()
Foo()

before

 WARN 
Identifier 'Foo' is not in snake case
at func.ab:1:5

We recommend using snake case with either all uppercase or all lowercase letters for consistency.
To disable this warning use 'allow_camel_case' compiler flag

1| fun Foo() {
2|     echo "Hello"
 WARN 
Identifier 'Foo' is not in snake case
at func.ab:1:5

We recommend using snake case with either all uppercase or all lowercase letters for consistency.
To disable this warning use 'allow_camel_case' compiler flag

1| fun Foo() {
2|     echo "Hello"
───────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
Hello
Hello

after

 WARN 
Identifier 'Foo' is not in snake case
at func.ab:1:5

We recommend using snake case with either all uppercase or all lowercase letters for consistency.
To disable this warning use 'allow_camel_case' compiler flag

1| fun Foo() {
2|     echo "Hello"
───────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
Hello
Hello

``